1. Field of the Invention
The present invention relates to a system and method for monitoring the input beam of a phase conjugated master oscillator power amplifier (PC MOPA) and shutting down the PC MOPA when its operation becomes multi-mode to prevent catastrophic damage of the MOPA components.
2. Description of the Prior Art
A master oscillator power amplifier (MOPA) architecture is known to be used in laser systems, such as solid-state laser systems, and provides a convenient and powerful way to scale the brightness of such laser systems. Solid-state MOPAs are known to include a master oscillator and an optical amplifier, known as a zig-zag amplifier. A primary laser signal from the master oscillator is amplified by the zig-zag amplifier to provide a relatively high average power output signal. The master oscillator normally includes a diode pumped, solid-state medium, a total reflector and an outcoupler for providing a primary laser signal. Examples of such solid-state lasers are disclosed in U.S. Pat. Nos. 3,679,999; 4,730,324; 4,852,109; 4,949,346; 4,984,246; 5,271,031; 5,305,345; 5,317,585; 5,351,251; 5,646,773; 5,555,254 and 5,307,430.
In order to improve the performance of such laser systems, phase conjugated MOPAs (PC MOPA) have been developed, for example, as disclosed in U.S. Pat. Nos. 4,734,911 and 5,555,254.
For solid-state lasers, the use of PC MOPAs has led to kilowatt class lasers with near diffraction limited beam quality. In most solid-state PC MOPA lasers, the phase conjugation medium is known to be a stimulated Brillouin scattering cell (SBS), for example as disclosed in commonly owned copending patent application Ser. No. 08/697,649 filed on Aug. 28, 1996, entitled "Ozone Compatible Stimulated Brillouin Scattering Materials, by Hagop Injeyan and Randall J. St. Pierre. Such SBS cells are known to provide outstanding performance in response to a single longitudinal mode input beam but suffer optical breakdown at modest energies with multi-longitudinal mode lasers. This breakdown can cause severe aberrations in the SBS medium leading to catastrophic optical damage of the MOPA components. Thus, there is a need for monitoring and shutting down lasers when the input beam to the PC MOPA is no longer single mode.